Re: [PATCH] tree-outof-ssa: fix unsignedp uninitialized in insert_value_copy_on_edge

On Thu, Aug 13, 2026 at 1:39 PM Jeffrey Law
<[email protected]> wrote:
>
>
>
> On 8/12/2026 10:27 PM, Sam Price wrote:
> > insert_value_copy_on_edge declares 'unsignedp' uninitialized and sets it
> > only as a side effect of an argument to gcc_assert.  When src_mode and
> > dest_mode differ it is then passed to convert_modes, which uses it to
> > choose between zero and sign extension.
> >
> > gcc_assert does not always evaluate its argument.  With GCC_VERSION
> > < 4005 and without assert checking, gcc/system.h defines
> >
> >    #define gcc_assert(EXPR) ((void)(0 && (EXPR)))
> >
> > so promote_ssa_mode is never called and unsignedp is passed to
> > convert_modes indeterminate, selecting zero or sign extension at random.
> > GCC_VERSION is __GNUC__ * 1000 + __GNUC_MINOR__, so any host compiler
> > reporting less than 4.5 takes this branch.
> >
> > Relying on a side effect in gcc_assert is against the coding
> > conventions regardless of which expansion is in use.  Call
> > promote_ssa_mode outside the assert, as the same file already does in
> > get_temp_reg, and keep the assert for the invariant alone.  unsignedp is
> > initialized for the !REG_P path, where promote_ssa_mode is not called.
> >
> > Signed-off-by: Sam Price<[email protected]>
> > Assisted-by: Claude (Anthropic)
> > ---
> >
> > Found while getting gcc.c-torture running against a MicroBlaze port to
> > LLVM, comparing the two compilers over the suite.
>
> So I don't think is necessarily as simple as it might appear.  In
> particular what is the right value for the !REG case.    I would suggest
> one of two paths to make sure we're safe WRT the unsignedp initialization.
>
> First, make the call to promote_ssa_mode unconditionally.  The worry
> here is that it may not be ready to handle non-REG cases.
>
> Second, in the src_mode != dest_mode code, assert REG_P (dest).
>
> Given the change affects generic code it would need to be bootstrapped
> and regression tested on one of the primary targets (x86, aarch64 linux
> being the most common).

So the problem will only show up with --disable-checking and a non GCC build.
Since GCC before 5.4.0 can't build GCC at this point. So maybe the
best way forward is either removing assert checking or just changing
the non-GCC gcc_assert case into the assert checking case.
I filed https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126838 and
Richard B. sugguests we remove --disable-checking. I tend to agree
with him.


Thanks,
Andrea

>
>
> Jeff